The fear of the Lord, John Murray explains, has two aspects. Terror and dread of God's holiness is the first aspect. Veneration and honor of God's majesty is the second. We moderns are happy to go along with Murray on the second aspect, but, to our detriment, we have tried to jettison the first. Murray writes,
It would be unnecessary to adduce the evidence establishing so obvious a distinction were it not the case that these two meanings of 'fear' enter into the concept of the fear of God. There is the dread or terror of the Lord and there is the fear of reverential awe. There is the fear that consists in being afraid; it elicits anguish and terror. There is the fear of reverence; it elicits confidence and love.
Both aspects of the fear of God must be part of the Christian's relationship to God...Because the justified Christian continues to sin, dread and terror of God cannot be absent:
Is it proper to be afraid of God? The only proper answer is that it is the essence of impiety not to be afraid of God when there is reason to be afraid.
Our sin is that reason:
The saint of God is not free from sin. He knows that sin is displeasing to God and he is keenly sensitive to the demands and judgment of his holiness. It is within this frame of thought and of feeling that we shall have to interpret those New Testament injunctions which never cease to have relevance to the believer during his sojourn here: 'Work out your own salvation with fear and trembling; for it is God who worth in you both to will and to do of his good pleasure (Philippians 2:12, 13); 'And if you call on the Father who without respect of persons judgeth according to each man's work, pass the time of your sojourning in fear' (1 Peter 1:17); 'Be not high-minded, but fear; for if God spared not the natural branches, neither will he spare thee' (Romans 11:20, 21). Humility, contrition, lowliness of mind are the essence of godliness. And the dispositional complex which is characterized by these fruits of the Spirit is one that must embrace the fear and trembling which reflect our consciousness of sin and frailty.
The rest of Murray's thoughts on the fear of God, which form the final chapter of his book on ethics called Principles of Conduct, can be read here.